1. Field of the Invention
The present invention relates to an optical scanning device, particularly an optical scanning device of the type including a minute mirror supported by two torsion bars and performing torsional vibration about the bars, and an image forming apparatus using the same.
2. Description of the Background Art
It has been customary with an optical scanning device to use a polygonal mirror, galvanometer mirror or similar deflector for deflecting an incident light beam. The problem with this kind of deflector is that deflecting speed is limited because of the durability of bearings, heat and noise and cannot meet the increasing demand for higher resolution and higher printing speed.
Today, optical scanning devices using micromachining are under study and expected to implement desirable optical writing devices for digital copiers, laser printers and other image forming apparatuses or reading devices for barcode readers and scanners. Japanese Patent Nos. 2,722,314, 3,011,144 and 2,924,200, for example, each disclose a particular system in which a movable mirror and torsion bars supporting it are formed integrally with each other by use of a silicon substrate. The movable mirror reciprocatingly vibrates due to resonance and therefore produces a minimum of noise while realizing high-speed operation. In addition, the movable mirror can be driven by a small torque and therefore with a minimum of power consumption. On the other hand, Japanese Patent Laid-Open Publication No. 11-218709 proposes correcting means for correcting the deformation of a movable mirror during vibration although the movable mirror is not formed integrally with torsion bars. Further, Japanese Patent Laid-Open Publication No. 4-86616 teaches a member for preventing a movable mirror from vibrating due to deformation when the direction of vibration is switched.
Although the movable mirror produced by micromachining has the advantages described above, it must be configured and dimensioned in accordance with resonance frequency and must be light enough to increase amplitude for a small torque. The mirror is therefore thin and apt to deform during vibration. Because the mirror performs sinusoidal vibration, negative acceleration increases with an increase in swing angle, resulting in an inertial force counter to rotation indirection. Consequently, the edge portions of the mirror positioned at both sides of torsion bars deform in the direction perpendicular to the torsion bars, varying the direction of beam deflection and thereby making scanning speed irregular on a line to be scanned. Further, the curvature of the deformed mirror shifts a beam focusing point.
Japanese Patent Nos. 3,011,144 and 2,924,200 mentioned earlier each disclose a particular system in which a movable mirror is formed with movable electrodes at its edges positioned at both sides of torsion bars while stationary electrodes each are positioned to face one of the movable electrodes. Electrostatic attraction is caused to act between the movable mirrors and the stationary mirrors for thereby driving the mirror. The electrodes are configured in the form of comb teeth and combined to have a broad area each, so that the electrostatic attraction is intensified. Generally, to provide the mirror with initial displacement, a step is formed between each movable electrode and stationary electrode adjoining it. Therefore, when the mirror is tilted in one direction, one of the movable electrodes is moved away from the associated stationary electrode. As a result, only the electrostatic attraction available with the other stationary electrode acts on the mirror. The system therefore fails to effectively use electrostatic attraction to act between the two movable electrodes and the two stationary electrodes.
IBM J. Res. Develop Vol. 24 (1980) teaches an optical scanning device in which a mirror base is supported by two torsion bars aligning with each other and is caused to perform torsional vibration about the torsion bars by electrostatic attraction, which acts between the mirror base and an electrode facing it. This optical scanning device, which is produced by micromachining, is simpler in structure than the conventional optical scanning device using a polygonal mirror to be driven by a motor and can be produced by a semiconductor process. The optical scanning device can therefore be easily reduced in size and cost. Further, the device has a single reflection surface and is therefore free from irregularity in accuracy particular to a plurality of reflection surfaces. In addition, the device is feasible for high-speed operation because of reciprocating scanning.
An electrostatic drive, torsional vibration type of optical scanning device is proposed in each of The 13th Annual International Workshop on MEMS 2000, PP. 473-478 and MEMS 1999, pp. 333-338. The scanning device of the type proposed includes an electrode facing the end face of a mirror base such that the electrode does not overlap the vibration range of the mirror base, so that the swing angle of the mirror base is increased. More specifically, the mirror base is implemented by a 20 μm thick, silicon base and plays the role of a movable electrode. Electrostatic attraction is caused to act between the mirror base or movable electrode and a stationary electrode that faces the end of the mirror base at a small distance. The two electrodes are formed at the same position. Particularly, the device proposed in MEMS 2000 uses the minute asymmetry of a structural body derived from a fabrication process to provide the mirror base with an initial moment for the startup of drive with respect to a twist axis. The device proposed in MEM 1999 has a think metal film or electrode for startup positioned in a plane perpendicular to a drive electrode.
Further, a trial, vibratory mirror chip using a thin film mirror is described in Optical MEMS 2000 and includes a circular frame. Polysilicon with a tensile stress is formed on the frame. An electrode resembling comb teeth is formed at the extension of the frame parallel to a torsion bar. An upper and a lower electrode cause the torsion bar to twist.
It is a common practice with a torsional vibration type of optical scanning device implemented by micromachining to form a mirror base by etching a silicon substrate through by dry etching. The mirror base is several ten micrometers. For example, the optical scanning device described in MEMS 2000 uses a 30 μm thick mirror base with the maximum area of 1.5 mm2. Likewise, MEMS 1999 uses a 20 μm thick mirror base with the maximum area of 3 mm2. Even such a thin mirror base is sometimes required to be dimensioned several millimeters at each side, depending on the configuration of a beam issuing from a light source or a required beam diameter on a surface to be scanned.
The swing angle of a mirror may be expressed as:θ=Trq×K(ω,δ)/I  Eq. (1)where I denotes the moment of inertia of the mirror, Trq denote a drive torque, ω denotes an angular velocity, δ denotes the viscous resistance of a space in which the mirror vibrates, and K(ω,δ) denotes a coefficient of vibration.
The moment of inertial I of the mirror may be produced by:
                                                        I              =                            ⁢                                                M                  ⁡                                      (                                                                  a                        ^                        2                                            +                                              b                        ^                        2                                                              )                                                  /                12                                                                                        =                            ⁢                              ρ                ⁢                                                                  ⁢                                                      tab                    (                                          a                      ^                      2                                        )                                    /                  12.                                                                                        Eq        .                                  ⁢                  (          2          )                    where M denotes the weight of the mirror, ρ denotes density, and b, a and t respectively denote the width, length and thickness of the mirror.
It will therefore be seen that to increase the swing angle of the mirror, the weight and therefore the moment of inertia of the mirror should only be reduced.
On the other hand, the resonance frequency f of the mirror may be expressed as:f=1/2π√(k/I)  Eq. (3)where k denotes the coefficient of torsional elasticity of a torsion bar.
Assuming that the torsion bar has a width c, a height t and a length L, then the coefficient of torsional elasticity k may be produced by:k=βtc^3E/L(1+ν)  Eq. (4)where β denotes the coefficient of a sectional shape, E denotes a Young's modulus, and ν denotes a Poisson's ratio.
As the above Eqs. (3) and (4) indicate, the resonance frequency of the mirror can be increased if the coefficient of torsional elasticity of the torsion bar is increased by increasing the sectional area of the torsion bar or by reducing the length of the same or if the mirror is reduced in weight to reduce the moment of inertia.
Reducing the weight of the mirror and therefore the moment of inertia is effective to increase the swing angle and operation speed. Particularly, reducing the weight of the mirror is essential for reducing the weight of the mirror as structural means for increasing the swing angle. However, if the thickness of the mirror is reduced for reducing the weight while guaranteeing a size required of a mirror base, then the mirror base deforms in the case of high-speed drive and cannot easily maintain its surface configuration constant, causing the beam configuration and focus to vary. Moreover, it is difficult to accurately control the thickness of the mirror on a production line; any irregularity in thickness directly translates into irregularity in resonance frequency.
As for the trial optical scanner described in Optical MEMS 2000, the size of the mirror or thin film available at the present stage of development is 1 mm or less. Therefore, should such a thin film be directly applied to a large mirror, the thin film would deform in the event of high-frequency operation or the frame supporting the thin film would deform due to the tensile stress of the thin film.